Article pubs.acs.org/cm
Quantitative Correlation between Carrier Mobility and Intermolecular Center-to-Center Distance in Organic Single Crystals Yoonkyung Park,† Kyung Sun Park,† Byeongsun Jun,‡ Yong-Eun Koo Lee,† Sang Uck Lee,‡ and Myung Mo Sung*,† †
Department of Chemistry, Hanyang University, Seoul 04763, Korea Department of Applied Chemistry and Department of Bionano Technology, Hanyang University, Ansan, Gyeonggi 15588, Korea
‡
S Supporting Information *
ABSTRACT: Charge transport properties of organic semiconductors critically depend on their molecular packing structures. Controlling the charge transport by varying the molecular packing and understanding their structure−property correlations are essential for developing high-performance organic electronic devices. Here, we demonstrate that the charge carrier mobility in organic single-crystal nanowires can be modulated with respect to the intermolecular center-to-center distance by applying uniaxial strain to the cofacially stacked crystals. Monotonic changes in charge carrier mobility (from 0.0196 to 19.6 cm2V−1s−1 for 6,13-bis(triisopropylsilylethylnyl) pentacene (TIPS-PEN)) were observed under a wide range of strains from −16.7% (compressive) to 16.7% (tensile). Furthermore, the measured values of charge carrier mobility were in good agreement with theoretical calculations based on charge localized hopping theory. These results provide a definitive relationship between intermolecular packing arrangement and charge transports, which enables a huge improvement in charge carrier mobility for organic single-crystal materials.
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INTRODUCTION Tremendous progress toward high-performance organic electronics has been achieved to date by increasing charge carrier mobility through rational designs of organic molecules, crystallinity/morphology control in organic materials, and optimization of device structures.1−5 In addition, much attention has been drawn to study charge transport in organic semiconductors in order to find fundamental routes to develop high-performance organic electronics based on an understanding of the intermolecular interactions and the structure− electrical property relationships.6−10 Organic semiconductors are composed of π-conjugated small molecules or polymers that are loosely bound by van der Waals forces.11−13 This structure results in materials that are inherently flexible and crystallize at low-temperatures; these properties are advantageous for producing inexpensive large-scale flexible electronics.14−16 In organic semiconductors, charge carriers are typically transported between neighboring molecules; therefore, the intermolecular packing arrangement, which is characterized in terms of π−π stacking distance and extent of π−π overlap, plays an © 2017 American Chemical Society
important role in the intrinsic charge transport rate, i.e., the carrier mobility.17−20 Importantly, the intermolecular packing arrangement can be altered by internal/external lattice strain because of the weak intermolecular interactions, which allows for modulation of the organic charge transport in the crystal structures.21−28 Understanding the correlation between molecular packing structures and charge transport properties would be useful for improving charge carrier mobility of organic semiconducting materials. The effects of strain on organic charge transport behavior have been studied by measuring the strain-induced electrical properties (field-effect mobility or electrical conductivity) of various organic flexible/stretchable devices.21−26,29−34 Singlecrystal organic semiconducting materials should be used for reliable studies on the strain effects because they have longrange ordered and well-defined molecular packing structures Received: February 27, 2017 Revised: April 17, 2017 Published: April 21, 2017 4072
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with no grain boundaries.21−26 Note that the strain-induced electrical properties in polycrystalline organic thin films are mostly described by the response of grain boundaries rather than that of the materials themselves.29−34 Studies using singlecrystal organic semiconductors have provided several important results with respect to the structure−electrical property relationships.21−26 An increase in the field-effect mobility of single-crystal 6,13-bis(triisopropylsilylethylnyl) pentacene (TIPS-PEN) was observed by inducing internal lattice strain during a crystal growth process to cause the greater orbital overlaps of adjacent molecules.23 Another study demonstrated that the carrier mobility of strained copper phthalocyanine (CuPc) single-crystal nanowires was higher than that of the straight counterparts because the augmented intermolecular interactions in the strained crystals lead to more efficient charge transport.24 Still, another study observed the charge transport modulation in single-crystal rubrene field-effect transistors by applying uniaxial strains to an elastomeric substrate containing the crystal film (wrinkling the conducting channels).25 The wrinkled crystals were subject to both local tensile and compressive strains, which resulted in an increase/decrease in the field-effect mobility depending on the net local strain at the dielectric/semiconductor interface of the wrinkled rubrene transistors. Very recently, a field-effect mobility increase of 70% was reported in a single crystal benzodithiophene derivative under 3% uniaxial compressive strain.26 This performance was ascribed to suppressed dynamic disorder due to the restricted vibration of the molecules in the compressed crystal structure. That study, however, was limited to very low strains because of crystal cracking and device failure, so only a small change in intermolecular distance was observed in order to define the structure−electrical property relationships. Therefore, it is necessary to investigate the charge transport behavior under a large range of strains, clarifying the structure−property relationships. A quantitative relationship between charge carrier mobility and crystal structure would be useful for understanding and improving charge transport properties in practical organic electronic devices. Here, we experimentally demonstrate a quantitative relationship between charge carrier mobility and intermolecular centerto-center distance in organic single crystals, and we further confirm the relationship using density functional theory (DFT) calculations based on the charge localized hopping theory.35−38 Organic single-crystal nanowires were employed as an ideal model due to their ultrahigh flexibility and anisotropic carrier transport along the nanowire long axis. These nanowires were used to efficiently study the intrinsic charge transport behavior even under a large strain without device failure. An array of parallel organic single-crystal nanowires printed on a flexible substrate was used for measuring strain-induced charge carrier mobility under a wide range of tensile or compressive strains. The strain was applied by bending the substrate containing nanowires along the nanowire long axes in order to manipulate the intermolecular center-to-center distance of the molecules. A strong correlation between the intermolecular center-to-center distance and the charge carrier mobility was observed. The measured charge carrier mobility was in good agreement with that from DFT calculations even under a strain as large as ±16.7%. These results demonstrate a definitive relationship between crystal structure deformation and electronic properties in single-crystal organic semiconductors.
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RESULTS AND DISCUSSION Organic single-crystal nanowires were used for studying the correlation between charge carrier transport and intermolecular center-to-center distance. Two organic semiconductors, TIPSPEN and N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8), were employed as representative π-conjugated p-type and n-type organic materials due to their good flexibility and chemical stability (See molecular structures in Figure 1a).19,39 TIPS-PEN or PTCDI-C8 single-crystal nanowire
Figure 1. (a) Molecular structures of TIPS-PEN (left) and PTCDI-C8 (right). (b) Well-defined diffraction spots in SAED patterns showing ab planes of triclinic TIPS-PEN (left) and PTCDI-C8 (right) crystal structures. The corresponding TEM images (insets) indicate that the nanowire long axis corresponds to [010] and [100] directions for TIPS-PEN and PTCDI-C8 crystals, respectively. The scale bar is 200 nm. (c) c-axis view of molecular packing structures of TIPS-PEN (left) and PTCDI-C8 (right) crystals, showing the π−π stacking direction of TIPS-PEN (b-axis) and PTCID-C8 (a-axis) nanowires.
arrays were fabricated using liquid-bridge-mediated nanotransfer molding (LB-nTM) methods.40,41 Briefly, inks were prepared by dissolving TIPS-PEN and PTCDI-C8 in 1,2,3,4tetrahydronaphthalene (1 wt %) and quinolone (0.1 wt %), respectively. Each ink solution was dropped onto a nanoscale line-patterned polyurethane acrylate (PUA) mold, and the ink selectively filled the nanoscale intaglio channels by discontinuous dewetting. At mild temperatures (15 mm), it can be assumed that the nanowires were tensiled/compressed toward only one direction, i.e., the nanowire long axes for both TIPS-PEN and PTCDI-C8. This strain can induce a change in intermolecular center-to-center distance between the two adjacent molecules of TIPS-PEN and PTCDI-C8, as illustrated in Figure 2c,d, respectively. Detailed illustrations of the variation in π−π stacking distance and extent of overlap in the crystal structures under unstrained and strained conditions are provided in Figure S2. The intermolecular center-to-center distance in single-crystal TIPS-PEN nanowires is 10.20 Å in the unstrained state. The distance decreased to 9.38 Å under −16.7% strain, while it increased to 11.10 Å under 16.7% strain. Similarly, the intermolecular center-to-center distance in PTCDI-C8 nanowires changed from 4.60 Å in the unstrained state to 3.83 and 4075
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Figure 4. (a, b) Normalized measured (black) and simulated charge carrier mobilities (red) of TIPS-PEN and PTCDI-C8 as a function of intermolecular center-to-center distance. Insets show the enlarged plots in selected ranges. (c) Molecular structures of TIPS-PEN and PTCDI-C8 with side view. θTIPS‑PEN and θPTCDI‑C8 are the angles between the strain direction and the principle axis, which is perpendicular to the π−π stacking direction for TIPS-PEN and PTCDI-C8, respectively.
charge hopping transport mechanism using the Q-Chem software package (version 4.2).35−38 The charge carrier mobility (μ) in organic semiconductors can be well described by the Einstein relation, μ = eD/κBT.36 The diffusion constant 1 D can be approximately evaluated by D = 2n ∑i ai 2kiPi , where the charge transfer rate (k) is expressed as 1/2 ⎡ (λ + ΔG°)2 ⎤ π 2π k = h λK T V 2exp⎢⎣ − 4λK T ⎥⎦ based on the localized B B charge hopping transport theory.20,28 Therefore, the charge carrier mobility is regulated by four variables, i.e., temperature (T), transfer integral (V), reorganization energy (λ), and intermolecular center-to-center distance (a). It is noted that the reorganization energy is a property of a material itself and is not significantly affected by lattice strain.19,20 At constant temperature, the variation in intermolecular center-to-center distance can affect the charge transport characteristics in organic materials. When the strain induces a change in intermolecular center-to-center distance of organic single-crystal nanowires, crystal structure deformations occur in the directions denoted as A and B, as illustrated in Figure 3a. In general, the deformation along the A and B directions mainly determines the extent of π−π overlapping and the π−π stacking distance, respectively. We calculated the transfer integral and charge carrier mobility of the lattice-strained TIPS-PEN and PTCDIC8 single crystals using the lattice-strained unit-cell parameters extracted from the DFT simulations (Table S1). The transfer integral is the electronic coupling strength, which is closely related to the degree of orbital overlapping.28 Thus, the transfer integral for each unstrained/strained TIPS-PEN and PTCDI-
respectively. The measured mobility values under other strain conditions are provided in Table S1. The I−V curves show Ohmic characteristics even under a large strain, suggesting that the mobility modulation in organic single-crystal nanowires was enabled over a wide range of strains without device failure. The measured carrier mobilities were not affected by device contact resistance according to our estimates of the contact resistances by the modified transfer length method (M-TLM), which is a typical method used for measuring the parasitic contact resistance in organic transistors.45,46 The estimated contact resistance was negligible compared to the total device resistance, and the contact resistances under different strain conditions were comparable (Figure S4) despite the significant differences in mobility under the two strain conditions, indicating that the measured mobility was modulated mostly by the intrinsic properties of the organic materials. In addition, the current was restored to its initial value when the applied strain was removed, indicating that the mobility modulation is reversible and reproducible. As a practical sensing application, a real human-motion strain detector was constructed using TIPSPEN single-crystal nanowire devices on a 5 × 1 cm2, 100 μmthick PU film. A highly sensitive and stable time-resolved current response was achieved under various external strains (Figure S5). A notable current response with respect to cyclical bending and unbending actions of the finger was observed, along with full recovery to the original state. A theoretical simulation was also performed to confirm the influence of intermolecular center-to-center distance on charge transport. The mobility was calculated by first-principles calculations using the DFT method based on the localized
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25.46) under a maximum applied compressive strain (−16.7%) and 0.02 ± 0.005 (cal. 0.007) under a maximum applied tensile strain (16.7%). Similarly, Figure 4b presents the normalized measured and calculated mobility of PTCDI-C8 nanowires as a function of intermolecular center-to-center distance, while the inset of Figure 4b shows the magnified plots of Figure 4b in a selected range from 4.60 to 5.37 Å. For PTCDI-C8 nanowires, the maximum normalized mobility was 3.45 ± 0.37 (cal. 2.87) at a distance of 3.83 Å, and the minimum value was 0.26 ± 0.06 (cal. 0.27) at a distance of 5.37 Å. Note that there is good quantitative agreement between the measured and calculated mobilities for both TIPS-PEN and PTCDI-C8 nanowires, indicating that the strain-induced mobility changes were influenced by localized charge hopping transport. Remarkably, the strain-induced enhancement in the normalized mobility (∼22 to 0.02) in TIPS-PEN was much larger than that in PTCDI-C8 (∼3 to 0.26). This can be explained by the fact that the charge carrier mobility is dependent on the molecular packing structure (Figure 4c).1,5,18 The principle axes (molecular long axes) in the crystal structures of both TIPSPEN and PTCDI-C8 are perpendicular to the π−π stacking direction and are oblique to the strain direction. As shown in Figure 4c, the principle axis of TIPS-PEN is less tilted to the strain direction than that of PTCDI-C8 so that the angle between the two axes of TIPS-PEN (θTIPS‑PEN) is larger than that of PTCDI-C8 (θPTCDI‑C8). Consequently, strain may affect the crystal structure deformation more dramatically on TIPSPEN than on PTCDI-C8. Indeed, the same strain induced larger variations in the charge carrier mobility for TIPS-PEN than for PTCDI-C8, probably due to a larger change in the π−π stacking distance (Table S3).
C8 single-crystal nanowires can be calculated using the intermolecular center-to-center distance (Figure S6). The transfer integral of TIPS-PEN increased exponentially from 0.00327 to 0.211 meV as the intermolecular center-to-center distance decreases from 11.10 to 9.38 Å. Such large changes in transfer integral within a single material may originate from the large difference in strain applied along the cofacially stacked molecules. Similarly, the transfer integral increased from 0.0520 to 0.237 meV as the distance decreased from 5.37 to 3.83 Å for PTCDI-C8 crystals. Because the rate of increase in the transfer integral is much larger than the rate of decrease in intermolecular center-to-center distance, the charge carrier mobility should increase with decreasing center-to-center distance, according to the Einstein relation. The 2D contour plots of hole and electron mobilities for TIPS-PEN and PTCDI-C8 are shown in Figure 3b,c, respectively, where different mobility values are expressed in different colors in the plane of π−π stacking distance and π−π overlapping extent change. All of the calculated mobility values under various strain conditions are summarized in the Table S2. As indicated by white dots in Figure 3b, the hole mobility of TIPS-PEN significantly increased when the intermolecular center-to-center distance became shorter. The calculated mobility was 1.20 cm2V−1s−1 at an intermolecular center-to-center distance of 10.20 Å (0% strain), while it changed from 0.00962 to 28.5 cm2V−1s−1 when the intermolecular center-to-center distance varied from 11.10 to 9.38 Å. Similar behavior was observed for PTCDI-C8 crystals as shown in Figure 3c. The electron mobility of PTCDI-C8 was 0.453 cm 2 V −1 s −1 at an intermolecular center-to-center distance of 4.60 Å (0% strain). The mobility of PTCDI-C8 varied from 0.121 to 1.30 cm2V−1s−1 as the intermolecular center-to-center distance changed from 5.37 to 3.83 Å. Note that the color changed drastically in the vertical direction (by changing the stacking distance), while the color changed only slightly in the horizontal direction (by changing the overlapping extent). The observed carrier mobility changes were mainly caused by changes in π−π stacking distance, which is illustrated clearly in the plots of the calculated charge carrier mobilities of the two crystals by varying π−π stacking distance, extent of π−π overlapping, or intermolecular center-to-center distance as a function of strain (Figure S7). Generally, the charge transport properties in organic semiconductors are greatly affected by both π−π stacking distance and extent of π−π overlapping, which strongly depend on molecular packing structures. In the strained TIPS-PEN and PTCDI-C8, the mobility values were considerably influenced by π−π stacking distance because the strain axis is nearly parallel to their π−π stacking direction, resulting in small changes of π−π overlapping degree between the adjacent molecules. Therefore, the intermolecular center-tocenter distance considering π−π stacking distance and extent of π−π overlapping should be addressed for the strain-induced modulation of charge carrier mobility in common organic semiconducting crystals. Figure 4a presents the normalized average mobility (black line) from the 30 TIPS-PEN device measurements as well as the theoretical one (red line) as a function of intermolecular center-to-center distance, where the mobility was normalized with respect to the value of the unstrained state. Figure 4a (inset) shows the magnified plots of Figure 4a in the range of intermolecular center-to-center distance from 10.20 to 11.10 Å, corresponding to tensile strain states. The normalized measured mobility of the TIPS-PEN nanowires was 21.56 ± 4.45 (cal.
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CONCLUSION In conclusion, we demonstrated that the charge transport properties of TIPS-PEN and PTCDI-C8 single-crystal nanowires were altered by applying a wide range of strains, even without crystal damage and/or current flow limits. Our study suggests that a quantitative change in charge carrier mobility can be achieved with respect to the change in intermolecular center-to-center distance, the degree of which greatly depends on the molecular packing structure of a crystal. A 3 orders of magnitude higher mobility (19.6 cm2V−1s−1) was obtained for TIPS-PEN crystals under a compressive strain of −16.7% when compared to the same crystals under a tensile strain of 16.7% (0.0196 cm2V−1s−1). Furthermore, the experimental mobility values were in good agreement with the theoretical values obtained by calculation based on charge localized hopping transport. These findings may open up a way to exploit strain to produce organic semiconductors with high charge carrier transport for future electronics applications.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00827. Photo and scheme of the bending machine, measured bending radius, calibrated strain, front and side views of the TIPS-PEN and PTCDI-C8 molecular packing arrangements, intermolecular center-to-center distance of TIPS-PEN and PTCDI-C8, plots for widthnormalized total resistance versus the reciprocal channel 4077
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length for the device containing TIPS-PEN nanowires, current responses, transfer integrals, and simulated charge carrier mobility; tables of transfer integral and charge carrier mobility of TIPS-PEN and PTCDI-C8, hole mobility of TIPS-PEN and electron mobility of PTCDI-C8, and charge carrier mobility of TIPS-PEN and PTCDI-C8; materials, detailed experimental methods, and simulations (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (M.M.S.). ORCID
Myung Mo Sung: 0000-0002-2291-5274 Author Contributions
M.M.S. conceived of and designed the experiments. Y.P. performed the experiments and analysis. B.J. and S.U.L. contributed to theoretical analysis. K.S.P., Y.P., Y.-E.K.L., S.U.L., and M.M.S. cowrote the paper. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Nano·Material Technology Development Program (2012M3A7B4034985) and by Creative Materials Discovery Program (2015M3D1A1068061) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning. This work was also supported by a grant from the National Research Foundation of Korea (NRF), funded by the Korea government (MSIP) (No. 2014R1A2A1A10050257), and by the Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1401-05. K.S.P. acknowledges the support of TJ Park Science Fellowship from POSCO TJ Park Foundation.
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DOI: 10.1021/acs.chemmater.7b00827 Chem. Mater. 2017, 29, 4072−4079
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DOI: 10.1021/acs.chemmater.7b00827 Chem. Mater. 2017, 29, 4072−4079
